Snap on were the first ones to offer a wrench like this, they were called flank drive. As John pointed out they are kind of a speed wrench. You turn the bolt and without removing the wrench from the bolt flats you just lift up on the end of the wrench and it just slips over to the next flat on the bolt head. They are great to work with in tight spaces. Here's a link to a set of craftsmen that do the same thing http://www.usedcarssearch.net/detai...-Speed-Wrenches-Set-Of-6-58_201283650820.html
Thanks for the replies but I don't think it's a speed wrench: the tolerances are very tight and you have to have wrench on almost perfectly square or it won't go over the nut and then there's no wiggle or ratchet to be had. I'm still thinking its for worn or extreme torq applications but I still haven't had the opportunity to really try it out
Just googled the patent - interesting stuff, should have done it earlier. It's designed to load the fastener so it doesn't slip:
Aug. 3, 1954 F DIEBOLD v 2,685,219 OPEN-END WRENCH JAWS HAVING THRUST AND PULL SURFACES OF DIFFERENT CURVATURE Filed March 25, 1953 i w s 7; \!Q- s I l/ INVENTOE Frifiz Diebold.